The present application relates to aqueous fluoropolymer dispersions.
Fluoropolymers, that is, fluorine containing homo and copolymers of fluoroolefins such as tetrafluoroethylene, trichlorofluoroethylene and vinylidene fluoride are especially desirable in formulating coating compositions having exceptionally high requirements of durability, weatherability, mar resistance, general appearance, as well as chemical, ultraviolet and thermal stability. Such coatings, however, are typically thermoplastic coatings which require baking at elevated temperatures to permit coalescence of the fluoropolymer binder, since most of the commonly employed fluoropolymers are inherently crystalline materials and insoluble in most common organic solvents at ambient temperature. Thermoserring fluoropolymers, that is, fluoropolymers which contain functional groups capable of coreacting with conventionally utilized crosslinking agents such as aminoplasts or blocked isocyanates, are not as widely available nor frequently utilized. This is generally owing to the fact that olefinic fluoromonomers such as for example vinylidene fluoride do not readily copolymerize with other monomers which contain functional groups capable of imparting crosslinking capability to the resulting fluoropolymer. For example, fluoroolefin monomers do not readily copolymerize with acrylate monomers such as 2-hydroxyethyl acrylate in order to impart functional characteristics to the fluoropolymer.
Olefinic fluoromonomers such as chlorotrifluoroethylene, however, readily copolymerize with vinyl ethers. These vinyl ethers may or may not contain fluorine as a substituent. The copolymerization of an olefinic fluoromonomer with a functional vinyl ether such as hydroxybutyl vinyl ether to achieve a hydroxyl functional fluoropolymer capable of crosslinking with crosslinking agents as described above is a well known method of preparing thermosetting fluoropolymers. Yamabe et al U.S. Pat. No. 4,345,057 assigned to Asahi Glass Co. is directed to such a curable fluoropolymer. When prepared as an aqueous dispersion, however, for instance in working example 7 of the patent, conventional fluorosurfactant, ammonium perfluorononanoate, is utilized to stabilize the dispersion. Such fluorosurfactants typically have low surface energy, adhering well to fluoropolymers and facilitating stabilization of the dispersion. However, the fluorosurfactants generally do not contain functional groups which can participate in crosslinking, therefore, films prepared from the resulting fluoropolymers have less than optimum physical properties such as diminished humidity resistance. Using conventional surfactants such as the ammonium salt of ethoxylated nonyl phenol sulfate in place of the fluorosurfactants has further drawbacks. These materials tend to have high surface energy, not adhering well to fluoropolymers and resulting in less than optimum conversion of monomers to polymer as well as instability of the fluoropolymer dispersion.
In a similar vein, European patent application 341,716 describes the preparation of an aqueous fluoropolymer dispersion wherein fluorosurfactant is utilized as a dispersant and polyethylene glycol monovinyl ether is a comonomer.
European patent application 0 320 156 discloses an aqueous based fluoropolymer wherein the fluoropolymer is dispersed in water as an amine carboxylate and acts as dispersant while acrylic polymer is prepared as a dispersed phase. In order to make the aqueous fluoropolymer dispersant, hydroxyl functional fluoropolymer is reacted with anhydride to generate the half acid ester for neutralization to an amine carboxylate for dilution with water.
The ability to prepare thermosetting aqueous fluoropolymer dispersions with a surface active agent which does not contain fluorine and is capable of crosslinking in a thermosetting system, rather than with conventional surfactants or fluorosurfactants is desirable.